The present invention relates to combined cycle electric power plants, and more particularly to improved bypass valve/control valve interaction for a steam turbine, particularly useful in the operation of a steam turbine powered by steam from one or both of two gas turbine heat recovery steam generators.
In the generation of electric power by a combined cycle plant of the type described, both gas turbines and the steam turbine may be in service, one gas turbine and the steam turbine may be in service, or only one or both gas turbines may be in service.
In such a plant, the turbine operates normally with its steam inlet control valves wide open without throttling, and with the load being governed by the rate of steam generation. The steam pressure is permitted to slide within certain limits depending on the loading of the turbine, and accepts whatever steam is generated.
The operation of a plant of this type is limited to a minimum steam pressure and flow because of the requirements of the heat recovery steam generators, and is further limited to a maximum velocity of steam to minimize erosion of the steam generator tubes and reduce the probability of water carryover into the turbine which could damage the turbine blades. At the same time, it is desirable to minimize throttling of the steam turbine control valves to maintain optimum plant efficiency and stability. This presents certain problems in that the maximum steam velocity which can be permitted depends on the steam pressure and the rate of steam generation. For example, in one embodiment, with both exhaust heat recovery steam generators in service, the rate of steam generation required no throttling of the steam turbine control valves except in maintaining the minimum required pressure for a load range of up to approximately 25%. However, with only one exhaust heat recovery steam generator in service, the rate of steam generation required throttling in the load range of up to approximately the 48% load range to maintain the minimum required pressure, and from above 70% to 100% of the load range to limit the maximum steam velocity. The amount of throttling, of course, is also variable. For example, with one generator in service, the system can operate as low as in the neighborhood of 500 pounds pressure for a steam flow or load of approximately 70% maximum, but must operate at 600 pounds pressure for a steam flow or load of approximately 80% maximum in order to maintain the steam velocity below a predetermined maximum.
In a plant of the type described the rate of steam generation can change rapidly and substantially in the event one of the steam generators is either put in or taken out of service; and in this event the rate of steam generation either increases or decreases rapidly and it is desirable that the pressure/flow relationship is changed in response to such event, without creating excessive pressure for a particular loading or increasing the probability of water carryover to the turbine. In starting up a plant of the type described, it is desirable to be able to control the steam inlet or control valves of the steam turbine independent of the pressure/flow relationship. This can be accomplished by bypassing the generated steam to the condenser and then as the control valve is opened to either accelerate the turbine or increase the load, the bypass valve can be modulated in order to maintain the proper pressure flow relationship to the turbine. Any minimum pressure flow relationship control after the bypass valve is closed can be maintained by the steam turbine control valves. Thus, the maximum velocity of the steam can be limited while still maintaining optimum efficiency. During a decrease in loading or speed, it is desirable that none of the steam is bypassed to the condenser unless the pressure/flow relationship becomes excessive. Therefore, it is desirable for the control valves to maintain control of the pressure/flow relationship with the bypass valve closed.
A sudden decrease or increase in pressure, depending on the type of contingency can trip the steam generator or turbine, unless such condition is remedied quickly by either preventing or causing a bypassing of the steam to the turbine. Therefore the bypass valve should be able to respond quickly to such change regardless of the particular pressure/flow condition prior to such change. In a system of the type where a transfer of control of the pressure/flow relationship occurs at a particular transition point would not necessarily react quickly enough for certain contingencies.
When the bypass valve is closed, any further valve restriction to maintain a proper minimum pressure/flow relationship must of necessity be controlled by the control valve or the rate of steam generation; but, even under these conditions the bypass valve should respond quickly to contingencies requiring responses.
Also, in stand alone steam turbine electric power plants of the sliding pressure type with a low and a high steam generation rate, which are often unattended a similar type of steam pressure/flow control can be desirable.